1. Field of the Invention
The present invention relates to a projection television device to obtain a wide screen image without difficulty, and more particularly to a projection television device to achieve an image with excellent quality.
2. Description of the Background Art
FIG. 6 schematically illustrates a general construction of a projection television device including three projection cathode-ray tubes for red image, green image and blue image. The projection television device of FIG. 6 includes a projection cathode-ray tube for red 1a, a projection cathode-ray tube for green 1b, a projection cathode-ray tube for blue 1c, projection lenses 2a, 2b and 2c and a screen 3. FIG. 7 is a cross-sectional representation of structures of phosphor layers used for a projection television device in a background art. Phosphor layers 4a, 4b and 4c shown in FIG. 7 each have an almost uniform thickness. Other reference characters of FIG. 7 correspond to those of FIG. 6.
The background-art projection television device has an arrangement, as shown in FIG. 6, where the projection lens 2b is positioned at center in front of the screen 3, the projection lens 2a is positioned at one side of the projection lens 2b and the projection lens 2c is positioned at the other side of the projection lens 2b, both being tilted to the screen 3, and the projection cathode-ray tube for green 1b, the projection cathode-ray tube for red 1a and the projection cathode-ray tube for blue 1c are positioned behind the projection lenses 2b, 2a and 2c, respectively. In practice, the phosphor layers 4a, 4b and 4c are considerably thin and almost uniformly coat the inner faces of display window glasses (not shown) provided on respective surfaces of the projection cathode-ray tube for red 1a, projection cathode-ray tube for green 1b and the projection cathode-ray tube for blue 1c.
Now, an operation of the background-art projection television device will be discussed. Referring to FIG. 7, the phosphor layer 4a is irradiated by an electron beam within the projection cathode-ray tube for red 1a to draw a red image on the phosphor layer 4a. Similarly, the phosphor layer 4b is irradiated by an electron beam within the projection cathode-ray tube for green 1b to draw a green image on the phosphor layer 4b, and the phosphor layer 4c is irradiated by an electron beam within the projection cathode-ray tube for blue 1c to draw a blue image on the phosphor layer 4c. Referring next to FIG. 6, the red image on the projection cathode-ray tube for red 1a is projected on the screen 3, being magnified through the projection lens 2a. The green image on the projection cathode-ray tube for green 1b is projected on the screen 3, being magnified through the projection lens 2b. The blue image on the projection cathode-ray tube for blue 1c is also projected on the screen 3, being magnified through the projection lens 2c.
In the background art, however, this projection television device causes such phenomena as follows:
(1) FIG. 8 is a graphical representation showing a relation between an area and a spot diameter of a phosphor layer. This graph shows a relative spot diameter, where the indication of 100% represents the spot diameter at the center portion of the phosphor layer receiving a perpendicular entry of an electron beam. As can be seen from FIG. 8, the spot diameter is larger at an area closer to the periphery of the phosphor layer. Taking the red image as an example for discussion, the electron beam enters the center portion of the phosphor layer 4a almost perpendicularly, but it enters the peripheral portion of the phosphor layer 4a with large angle of incident to make the spot diameter larger. Accordingly, the resolution at the peripheral portion of an image on the screen 3, which is obtained by projecting the image of the phosphor layer 4a, is worse than the resolution at the center portion of the image on the screen 3.
(2) FIG. 9 is a graphical representation of MTF characteristics, showing a relation between an area and a resolution (MTF) of the image on the screen 3. As can be seen from FIG. 9, the MTF at the peripheral portion of the image projected on the screen 3 is worse than the MTF at the center portion of the image projected on the screen 3. Taking the red image as an example for discussion, the projection lens 2a generally has a spherical aberration and fabrication variation. When the image of the phosphor layer 4a is projected on the screen 3 through the projection lens 2a, the loss in MTF is produced more at the peripheral portion of the projected image than at the center portion of the projected image due to the spherical aberration and fabrication variation.
(3) FIG. 10 is a graphical representation showing a relation between an area and a luminance (brightness) of the image projected on the screen 3. This graph shows relative luminance, where the indication of 100% represents the luminance of the image projected on the center portion of the screen 3. As can be seen from FIG. 10, the luminance at the peripheral portion of the image projected on the screen 3 is worse than the luminance at the center portion of the image projected on the screen 3. Taking the red image as an example for discussion, a capture range of projected light in an entrance pupil of the projection lens 2a is wide at its center portion and narrow at its peripheral portion. Accordingly, when the image of the phosphor layer 4a is projected on the screen 3 through the projection lens 2a, the loss in luminance is produced more at the peripheral portion of the image projected on the screen 3 than at the center portion of the image projected on the screen 3 due to the difference in capture range of the projection light from the phosphor layer 4a (the light of image emitted from the phosphor layer 4a)
These phenomena (1), (2) and (3) as discussed above similarly occur in cases of the green image and blue image.
(4) FIG. 11 illustrates images which are drawn on the phosphor layers 4a, 4b and 4c when an image having a pattern 7 shown in FIG. 12, as an example of image, is formed on the screen 3. As can be seen from FIG. 11, the images of the phosphor layers 4a and 4c each have a pattern which is drawn larger on a near side to the phosphor layer 4b and drawn smaller on a far side from the phosphor layer 4b, in contrast to the image of the phosphor layer 4b. This is because each of the images of the phosphor layers 4a and 4c on the near side to the phosphor layer 4b has a tendency to be projected smaller on the screen 3 since the projection cathode-ray tube for red 1a and the projection cathode-ray tube 1c are arranged at a large tilt with respect to the screen 3. For this reason, correction of the image on the screen 3 is made by drawing the image larger on the nearer side to the phosphor layer 4b.
There arises such unbalance as the images of the phosphor layers 4a and 4c are each drawn larger on the near side to the phosphor layer 4b and drawn smaller on the far side from the phosphor layer 4b. The images with this unbalance are accompanied by the loss in resolution as discussed in (2) and the loss in luminance as discussed in (3) when it is projected on the screen, being magnified through the projection lens 2a or 2c, to produce difference in resolution and luminance between both sides of the image on the screen 3.
Thus, in the background art, there are problems of the loss in resolution and luminance which is produced more at the peripheral portion of the image on the screen 3 than at the center portion of the image on the screen 3, due to the phenomena (1), (2) and (3) and of the difference in resolution and luminance between both sides of the image on the screen 3 due to the phenomenon (4).